This invention relates to moisture sensors for automotive vehicle windows such as windshields, and more particularly to control systems incorporating such sensors to operate vehicle accessories such as windshield wipers.
In one type of windshield moisture sensor, a light beam is directed into the windshield at an angle and the refracted/reflected portion of the beam is monitored by a photodetector which produces a moisture signal. When moisture is present on the window, a decreased amount of light is refracted/reflected to the photodetector, which reduces the amplitude of the moisture signal. The moisture signal is monitored by circuit means to evaluate whether moisture is present.
Such a moisture sensor typically further includes means for producing a reference level against which the moisture signal is compared. When the magnitude of the moisture signal drops below the value of the reference signal, the system actuates the windshield wipers. When a fixed reference level is provided, the degradation of the system components with age and the variations in their performances due to temperature and other environmental factors, results in a system where false wipes are frequent or no wipes may occur even when moisture is on the window.
In order to overcome the difficulties of the fixed reference signal systems, it has been proposed to provide a variable reference signal that will be modified in response to the same conditions that are causing changes in the moisture sensing system. The reference signal in such a system may be adjusted by a second detector, which does not receive light that has been affected by the moisture condition of the windshield but would otherwise be affected by the environmental conditions experienced by the primary detector. Such systems can require a considerable amount of calibration in order to produce similar signal characteristics. The two detectors furthermore do not necessarily respond to all environmental conditions in the same manner.
Another attempt to overcome the difficulties of the fixed reference signal system has been to provide a variable reference generating circuit which responds directly to the moisture signal when the windshield is in a dry condition, as occurs immediately after the wiper has passed the sensor, to establish a reference signal which adapts to changes in the condition of the system, the windshield and the wipers. While such adaptive reference value systems avoid some of the difficulties of the other systems, certain environmental influences, such as street lights, oncoming headlights, taillights, commercial signs and other filament-type light bulbs may adversely influence the reference level, which tends to adapt to these environmental influences in a manner that may cause the reference signal to increase above the moisture signal even when the windshield is dry, thus producing false wipes. Such known adaptive reference value systems typically employ a peak sample and hold circuit to store a reference value at the moment the windshield wiper passes over the sensor area, which is presumed to be the driest condition of the windshield. Such systems are susceptible to recording an erroneous reference signal as a result of reflections and other interference caused by passage of the wiper blade over the sensor area.
Another problem encountered by prior windshield moisture sensor systems in general, and adaptive reference value systems specifically, is that the sensor may erroneously interpret a dry contaminant film, such as dirt or salt, that may be deposited on the windshield during a rain condition, as moisture, and continue to operate the wipers even after the windshield is dry and even though the wipers are clearly not having an effect on the film.